1. Field of the Invention
This invention relates generally to the cell structure and fabrication process of power semiconductor devices. More particularly, this invention relates to a novel and improved cell structure and improved process for fabricating a trench MOSFET with trench source contact having copper wire bonding.
2. The Prior Arts
With the increasing demands of better performance, the trench MOSFET has been required to have a lower spreading resistance, smaller size and reducing fabricating cost. As shown in FIG. 1, a cell MOSFET structure of prior art (U.S. Pat. No. 6,066,877) with relatively thick metal layer is disclosed to reduce the distributed resistance of the source metal layer. Trench MOSFET 101, as illustrated, is formed on an N+ substrate 100 on which an N-doped epitaxial layer 102 is grown. Inside said epitaxial layer 102, individual transistor cells 101A-101D are formed between trench gates 102A-102E, respectively. In order to connect N+ source regions, which are formed adjacent to trench gates 102A-102E, a thin metal layer 103, typically Al, is deposited at the top surface of epitaxial layer 102. P-body regions within each cell 101A-101D, is shorted to N+ source regions also by means of said thin metal layer 103. In passivation layer 104 over thin metal layer 103, an opening 105 is formed for the deposition of thick metal layer 110. In this structure, said thick metal layer 110 consists of a Zn adhesion sublayer 110A, a thick Ni sublayer 110B, and a gold surface sublayer 110C. Au wire 107 is bonded to the top surface of metal layer 110. According to the prior art, the thick metal layer provides essentially no resistance between the transistor cells, and therefore the overall spreading resistance is dramatically reduced.
However, this “low resistance” performance is based on the sacrificial of other important characteristics. First of all, the additional Ni and Au deposition is quite expensive for the aspect of fabricating cost, and second, it is not really necessary because the additional Ni and Au deposition has a low cell density (<100M/in2) with planar source contact, resulting in a huge waste of area and the corresponding increasing of spreading resistance, which counteracts the advantages by depositing the thick metal layer.
Accordingly, it would be desirable to provide a new trench MOSFET cell structure with low spreading resistance, and at the same time, with small size and low fabricating cost.